The present invention relates to nuclear reactors and, more particularly, to a method of repairing a boiling-water nuclear reactor.
Nuclear reactors are conservatively specified to minimize any risks from the hazardous materials involved in their use. Reactor vessel walls are several inches thick and the strongest materials are used for reactor components. Nonetheless, contingencies are required for failure as components are subjected to extreme stress for decades. These contingencies involve not only many layers of preventive systems, but also procedures for rectifying problems that arise.
Of concern herein are incore-housing-related defects. Incore instrumentation housings, referred to more simply as "incore housings", house the links between instrumentation used to monitor the core and the host system used to analyze the data collected by instrumentation. The housings are tubular and penetrate the reactor vessel bottom, to which they are welded. Incore-housing-related defects include defects in the housing itself, in the weld bonding the housing to the vessel bottom and in the vessel bottom in the vicinity of the housing. These defects can cause or lead to leaks from the reactor vessel. Accordingly, a method is required to address such defects.
Drastic approaches involve long-term shutdown of the reactor. The reactor could be replaced or removed, repaired, and reinstalled. These approaches are extremely costly, and alternatives are highly desirable. Some incore housing defects can be addressed by inserting a sleeve onto the incore housing to cover the defects and stop whatever leaking might occur. However, such patch approaches do not really address the defect, which can continue to grow due to further fatiguing. Furthermore, the weld between the sleeve and the housing can be a new source of defects.
More recently, a method has been developed for replacing an incore housing that minimally impacts reactor components other than those being replaced. This method involves removal of all fuel from the reactor and performing most steps under water to minimize radiation exposure. While providing a relatively permanent repair and while being relatively economical, this method involves a series of over 100 procedures, several of which could introduce new defects. Such defects may be serious enough to force repetition of the replacement method. This repetition is undesirably time consuming and expensive. What is needed is an improved method of effecting a relatively permanent repair of an incore housing defect that is relatively economical and, preferably, minimally disruptive of the reactor system.